The present invention relates to a new process for the manufacture of halonitrobenzenes by reacting halonitroanilines with alkanols and nitrosating agents in the presence of water and an acid at not less than 35.degree. C.
Houben-Weyl, Methoden der Organischen Chemie, Volume 10/3, pages 116 et seq. discloses that aromatic diazonium salts, in alcohols, can be converted to the corresponding aromatic hydrocarbons by heating; it is recommended to use as concentrated a solution of the diazonium salt as possible and to add to the solution from 5 to 10 times its volume of the alcohol. In the reaction, the alcohol is converted to the corresponding aldehyde and the resulting increase in aldehyde content prevents re-use of the unconverted alcohol. Depending on the structure of the diazonium salt, the reaction must either be carried out under anhydrous conditions or an 80 percent strength by weight aqueous ethanol solution can be used. Organic Reactions, Volume II, page 274 (Wiley, N.Y.) also teaches that whilst it is not absolutely essential to carry out the reaction under anhydrous conditions, the amount of water should be restricted to from about 5 to 10%.
In addition to the hydrocarbons, the phenol ethers corresponding to the alcohol used, and greater or lesser amounts of resin (Houben-Weyl, loc. cit., pages 123 and 124) are formed as by-products, especially if the reaction is carried out with alcohol diluted with water. The yield and purity of the end products of these processes are in most cases unsatisfactory, particularly in industrial operation. Thus, for example, the yield of end product is stated to be 46% when 2,4-dichloroaniline is used as the amine starting material and 53% when anthranilic acid is used as the amine starting material (Houben-Weyl, loc. cit., page 125). An article in Angewandte Chemie, 70 (1958), 211, discloses that instead of alcohols, ethers such as dioxane must be used to avoid the formation of by-products and to improve the yield of end product. Equally, it is possible, instead of using the aqueous diazotization solution, to isolate the diazonium salt itself and to react it with the alcohol (Sounders, "The Aromatic Diazocompounds" (E. Arnold & Co., London 1949), page 271). All these processes are unsatisfactory, particularly in industrial operation, in respect of economy and simplicity of operation and yield of end product.
An article in Science, 117 (1953), 379 - 380 discloses that the reaction of the benzenediazonium salt with an alcohol in most cases leads to the corresponding phenyl alkyl ether and not to the benzene derivative which remains after elimination of the diazonium group, or only to minor amounts of this derivative. This is also disclosed in H. Zollinger, Azo and Diazo Chemistry, Interscience Publishers, New York and London, 1961, page 141. Houben-Weyl also points out (loc. cit., page 124) that the decomposition of numerous diazonium salts by heating in ethanol results in replacement of the diazonium group by the ethoxy radical. According to this disclosure, replacement of the diazonium group by hydrogen requires certain reaction conditions, such as the addition of zinc, or irradiation with ultraviolet light. To achieve higher yields of benzenes from the reduction with alcohols, it is recommended to add alkalis or copper compounds or zinc compounds (loc. cit., pages 119 and 127). Houben-Weyl points out (loc. cit., page 128) that with increasing temperature the ratio of the two reaction products, namely the phenol ether and the hydrocarbon, shifts in favor of the former. Since the product mixtures are often difficult to work up, and the yield of the desired hydrocarbon is poor, the use of other reducing agents is recommended (Zollinger, loc. cit., page 168).
J. Amer. Chem. Soc., 72 (1950), 798, discloses that 2,6-dichloro-4-nitroaniline, ethanol and sodium nitrite can be reacted at the boil, in the absence of water and in the presence of concentrated sulfuric acid, to give 3,5-dichloronitrobenzene in a yield of 84%. A corresponding reaction to give the 3,5-dibromo derivative gives a yield of 91%. The process is unsatisfactory in respect of yield and purity of the end product and simple, reliable and economical operation, especially on an industrial scale.